What Wikipedia entry (to which "Wild West" redirects) begins thus: "The [TWO-WORD ANSWER] comprises the geography, history, folklore, and cultural expression of life in the forward wave of [FIRST WORD] expansion that began with English colonial settlements in the early 17th century and ended with the admission of the last mainland territories as states in 1912. A [SECOND WORD] is a zone of contact at the edge of a line of settlement."?
American frontier